The moon was formed from debris following a collision between Earth and an asteroid about the mass of Mars, according to a researcher who disputes current theories that a far larger asteroid was involved.

The smaller asteroid theory, first proposed in the 1970s, had fallen out of favor as physicists estimated a much larger impact would have been necessary to create Earth's companion.

However, new simulations taking advantage of computing advances and reviews of previous simulations show an object about the mass of Mars, striking Earth off-center, would have been enough to put the two in their current position, researcher Robin Canup said.

``We've really come full circle and it looks like the more simple original idea from the mid-70s,'' said Canup, of the Southwest Research Institute in Boulder, Colo. ``To me, this greatly increases my confidence in the impact theory.''

A smaller asteroid also eliminates the need for a second massive impact called for under some scenarios, Canup and Erik Asphaug of the University of California, Santa Cruz, report in Thursday's issue of the journal Nature.

Scientists have largely discredited other theories, including that Earth's gravity captured the moon as it strayed too close, or that the Earth and moon formed simultaneously. Problems with those theories include accounting for the moon's lack of iron, which remained on Earth while lighter material was flung into space under the asteroid-strike theory.

Al Cameron, the Harvard researcher who first proposed the Mars-sized asteroid theory, said Canup's simulation only deals with the initial impact, not the full formation, and assumes the material ejected into space is hard rock and not rubble.

Cameron said he also believes the Earth was only about two-thirds formed when the moon-forming asteroid struck, rather than almost fully formed as Canup believes.

Cameron, however, said he did not dismiss the new finding, ``it is just that the problem is very complicated and the definitive story has yet to be determined.''


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